The objective of the proposed work is to identify certain structural features of immunoglobulin molecules that can be used as probes in monitoring the genetic controls and restrictions governing the synthesis of antibody. At the present time the only useful probes available are in the form of ill-defined antigenic determinants. The current proposal seeks to establish a chemical or structural basis for the antigenic determinants, notably allotypic determinants, in question. We plan to identify and localize by fragment and peptide analyses where such antigenic determinants reside. A part of this effort will involve comparisons of proteins, peptides and antigens found on immunoglobulins of different but phylogenetically related species.